1. Field of the Invention
The present invention pertains to an impedance measuring apparatus, and in particular relates to an impedance measuring apparatus with which high-speed measurement is possible.
2. Discussion of the Background Art
Impedance measuring apparatuses that operate by the automatic balanced bridge method are an example of the prior art of impedance measuring apparatuses. Impedance measuring apparatuses that operate by the automatic balanced bridge method are characterized in that they cover a broad measurement frequency range and their measurement accuracy is good within a broad impedance measurement range.
The internal structure and operation of an impedance measuring apparatus that operates by the automatic balanced bridge method are described below. FIG. 1 is a drawing showing the internal structure of an impedance measuring apparatus that operates by the automatic balanced bridge method. The impedance measuring apparatus 10 in FIG. 1 comprises a signal source 200, a current-to-voltage converter 300, and a vector voltmeter 400 for determining the impedance of a device under test 100. The entire impedance measuring apparatus 10 is operated under the control of an operation control device CTRL1 (not illustrated), such as a CPU.
The device under test 100 is an element or a circuit having two terminals. Device under test 100 should have at least two terminals and therefore, it can be an element or a circuit with three or more terminals. In this case, two of the three or more terminals are used for the measurements. Device under test 100 is represented by “DUT”in FIG. 1. The point where device under test 100 is connected to a cable 510 and a cable 520 in FIG. 1 is referred to as the High terminal. Moreover, the point where device under test 100 is connected to a cable 530 and a cable 540 is referred to as the Low terminal.
The signal source 200 is the signal source that is connected to the first terminal of device under test 100 by cable 510 and generates measurement signals that are fed to device under test 100. Moreover, signal source 200 is also connected to vector voltmeter 400 by cable 510, cable 520, and a buffer 550 and feeds the measurement signals to vector voltmeter 400. The measurement signals are single sine-wave signals.
The current-to-voltage converter 300 converts the current that flows to device under test 100 and outputs the voltage signals to a buffer 560. Current-to-voltage converter 300 comprises a zero-phase detector 310, a narrow-band amplifier 600, a buffer 320, and a range resistor 330. Cable 530, zero-phase detector 310, narrow-band amplifier 600, buffer 320, range resistor 330, and cable 540 form a negative feedback loop 340.
The zero-phase detector 310 balances the current that flows to range resistor 330 and the current that flows to device under test 100 and outputs signals to narrow-band amplifier 600 such that the current that flows into the input terminals of zero-phase detector 310 through cable 530 will be brought to zero. When the current that flows to range resistor 330 and the current that flows to device under test 100 are balanced, the current at the Low terminal is kept at virtual ground.
Refer to FIG. 2. FIG. 2 is a drawing showing the internal structure of the narrow-band amplifier 600. Narrow-band amplifier 600 comprises a quadrature detector 610, a filter 620, a filter 630, and a vector modulator 640, and amplifiers, and amplifies the output signals of zero-phase detector 310 and outputs them to buffer 320. Narrow-band amplifier 600 resolves the output signals of zero-phase detector 310 into an in-phase component and an quadrature-phase component using phase detector 610, filters the in-phase component and quadrature-phase component by means of filters 620 and 630, modulates the filtered in-phase component and quadrature-phase component by means of vector modulator 640, and feeds the vector-modulated voltage signals to buffer 320.
The signal source 650 is a sine-wave signal source having the same frequency as the measurement signals. The output signals of signal source 650 are converted to cosine-wave signals by a phase-shift circuit 660. Moreover, a phase-tracking circuit 670 outputs the output signals of signal source 650 staggered by a pre-determined phase. A phase-tracking circuit 680 outputs the output signals of phase-shift circuit 660 staggered by a pre-determined phase.
The quadrature detector 610 comprises mixers 611 and 612. The output signals of phase-tracking circuit 670 are input to mixer 611. The output signals of phase-tracking circuit 680 are input to mixer 612. The sine-wave signals output by phase-tracking circuit 670 and the cosine-wave signals output by phase-tracking circuit 680 have the same frequency as the measurement signals and they are orthogonal to each other. Consequently, mixers 611 and 612 can orthogonally resolve the output signal of zero-phase detector 310 into an in-phase component and an quadrature-phase component.
The filter 620 is an integrator that comprises a resistor and that integrates the output signals of mixer 611. The filter 630 is an integrator that integrates the output signals of mixer 612.
The vector modulator 640 comprises mixers 641 and 642, and an adder 643. The output signals of signal source 650 are input to mixer 641. The output signals of phase-shift circuit 660 are input to mixer 642. The sine-wave signals output by signal source 650 and the cosine-wave signals output by phase-shift circuit 660 have the same frequency as the measurement signals, and they are orthogonal to each other. Mixer 641 modulates the output signals of filter 620 with the sine-wave signals that are output from signal source 650 and outputs these signals. Mixer 642 modulates the output signals of filter 630 with the cosine-wave signals output from phase-shift circuit 660 and outputs these signals. The voltage signals output from mixer 641 and the voltage signals output from mixer 642 are added by adder 643 and output to buffer 320.
As is also made clear from the above-mentioned description, the band that is temporarily amplified by narrow-band amplifier 600 is narrow but can correspond to a broad frequency range because the frequency of the output signals of signal source 650 are variable.
Refer to FIG. 1 again. Vector voltmeter 400 measures the output signal Edut of buffer 550 and the output signal Err of buffer 560. Control device CTRL1 calculates the vector ratio between signal Edut and signal Err that have been measured and further, calculates the impedance of device under test 100 from the calculated vector ratio and the resistance of range resistor 330.
Measurement of the gate oxide film thickness is one important measurement in the production of MOS devices formed on a semiconductor wafer. The gate oxide film thickness is an important parameter in determining the operating threshold of MOS-type devices. The gate oxide film thickness is measured by measuring the impedance of an MOS device, calculating the capacitance from the impedance measurement, and converting this calculated capacitance to the equivalent oxide film thickness using the dielectric constant. There are two problems with measuring the impedance of an MOS device with conventional impedance measuring apparatus 10.
The first problem is that when the frequency of the output signals of signal source 650 change, it takes time until the signals that are input to mixer 611 and the like stabilize. The frequency of the output signals of signal source 650 change in accordance with the measurement signals when the impedance of device under test 100 is measured at multiple frequencies. As previously mentioned, the output signals of signal source 650 are input to mixer 611 and the like through phase-shift circuit 660, phase-tracking circuit 670, or phase-tracking circuit 680. Conventional phase-shift circuit 660 comprises a PLL circuit. Moreover, phase-tracking circuits 670 and 680 comprise a all-pass filter or a high-passfilter. Consequently, when the frequency of the signals that are input change, it takes time until the output signals of phase-shift circuit 660, phase-tracking circuit 670, and phase-tracking circuit 680 stabilize. An impedance measuring apparatus measures the impedance while shifting to several frequencies. In this case, if the signal convergence time is prolonged, the convergence time of the output signals of current-voltage converter 300 will also be prolonged, delaying the start of measurement. In the end, the above-mentioned signal convergence time becomes a detrimental factor to high-speed measurement.
The second problem is the trade-off between measurement precision and circuit size. Impedance measuring apparatus 10 is adjusted to a open-loop phase shift of negative feedback loop 340 of 180° for stable operation of this negative feedback loop 340. Specifically, operation control device CTRL1 controls phase-tracking circuit 670 and adjusts the phase difference between the signals input to mixer 611 and the signals input to mixer 641. Moreover, operation control device CTRL1 controls phase-tracking circuit 680 and adjusts the phase difference between the signals input to mixer 611 and the signals input to mixer 641. Conventional phase-tracking converters 670 and 680 comprise circuits with passive components, such as resistors and capacitors. If phase-tracking circuits 670 and 680 are to precisely adjust the above-mentioned phase difference, they must have many combinations of resistors and capacitors. The increase in the number of components leads to an increase in the circuit size and a decrease in the mean-time-before-failure (MTBF). Moreover, resistors and capacitors are dependent on temperature, and an increase in the number of components invites an increase in measurement errors. On the other hand, when there is a reduced number of resistors and capacitors, the convergence time of negative feedback loop 340 is increased. In particular, when the impedance of an MOS device formed on a wafer is measured, it is measured through a switch matrix and probe cards, and the like, and therefore, the problem of convergence time becomes obvious. In addition, there are cases in which the phase shifts of phase-tracking circuits 670 and 680 are not the same and measurement error is generated.
There has been considerable progress in microfabrication technology for semiconductors in recent years, with a huge number of elements or circuits being formed on one wafer. While there has been an obvious increase in the number of elements that serve as the device under test, a corresponding increase in measurement time is not allowed. Moreover, the sacrifice of measurement precision for high-speed measurement is not acceptable. The realization of high-speed, high-precision impedance measurement is a very important problem in the semiconductor industry today.